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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) $ in Thousands	Mar. 31, 2021 USD ($) security	Sep. 30, 2020 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of debt securities with unrealized loss | security	38	51
Available-for-sale securities
Less than 12 months, unrealized gross losses	$ (3,554)	$ (5,833)
12 months or more, unrealized gross losses	(1,369)	(3,781)
Total unrealized gross losses	(4,923)	(9,614)
Less than 12 months, fair value	260,916	675,518
12 months or more, fair value	157,632	285,161
Total, fair value	418,548	960,679
Held-to-maturity securities
Less than 12 months, unrealized gross losses	0	(52)
12 months or more, unrealized gross losses	0	0
Total unrealized gross losses	0	(52)
Less than 12 months, fair value	0	6,853
12 months or more, fair value	0	0
Total, fair value	0	6,853
Less than 12 months, unrealized gross losses	(3,554)	(5,885)
12 months or more, unrealized gross losses	(1,369)	(3,781)
Total unrealized gross losses	(4,923)	(9,666)
Less than 12 months, fair value	260,916	682,371
12 months or more, fair value	157,632	285,161
Total fair value	418,548	967,532
Corporate debt securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(1,514)	(74)
12 months or more, unrealized gross losses	0	(405)
Total unrealized gross losses	(1,514)	(479)
Less than 12 months, fair value	125,191	45,875
12 months or more, fair value	0	24,596
Total, fair value	125,191	70,471
Asset-backed securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(77)	(5,481)
12 months or more, unrealized gross losses	(1,058)	(3,027)
Total unrealized gross losses	(1,135)	(8,508)
Less than 12 months, fair value	80,046	587,746
12 months or more, fair value	125,681	204,369
Total, fair value	205,727	792,115
Mortgage-backed securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(1,963)	(278)
12 months or more, unrealized gross losses	(311)	(349)
Total unrealized gross losses	(2,274)	(627)
Less than 12 months, fair value	55,679	41,897
12 months or more, fair value	31,951	56,196
Total, fair value	$ 87,630	$ 98,093

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